I. Trichohyalin
One of the major differentiation products of the inner root sheath and medullary cells of the developing hair follicle. Upon terminal differentiation in these tissues, the granules disperse, but the final fate and structure of TRHY appears to be site dependent: in the inner root sheath, the TRHY protein becomes enmeshed with the keratin intermediate filaments ("KIF") of the cells with an apparent periodicity of about 200 nm (range 100-400 nm) or 400 nm (range 200-500 nm); in the medulla, the protein forms amorphous deposits that are not organized in any specific way.
TRHY undergoes a series of calcium-dependent postsynthetic enzymatic modifications. For example, it becomes highly cross-linked to the KIF by way of N.sup..epsilon. -(.gamma.-glutamyl)lysine isodipeptide crosslinks which may be formed by the action of transglutaminases of the hair follicle cells. In addition, many of the arginine residues are desimidated to citrullines by the action of the enzyme peptidylarginine deiminase.
More recently, it has become clear that the expression of TRHY is not confined to the hair follicle. There is evidence showing that TRHY is expressed in the filiform papillae of dorsal tongue epithelium (Lynch, M. H., et al., J Cell Biol. 103:2593-2606(1986)), a region that undergoes a course of "hard" keratin differentiation related to that in the hair follicle. In addition, indirect immunofluorescence data indicate that TRHY is also expressed in modest amounts in the granular layer of newborn human foreskin epidermis, although whether it is expressed in interfollicular trunk epidermis is not yet clear.
Current physico-chemical data suggests that human, sheep and pig TRHYs are large proteins of apparent molecular weight of about 200 kDa (Fietz, M. J., et al., J. Cell Biol 110:427-436 (1990); O'Guin, W. M., et J. Invest. Dermatol. 98:24-32 (1992); Hamilton, E.H., et al., J. Invest. Dermatol 98:881-889 (1992)). For pig TRHY there is evidence that two components of about 220 and 200 kDa exist. Shadowed electron micrographs of native pig tongue TRHY reveal an elongated particle of about 85 nm with a small bead on one end.
II. Transglutaminase-3
Transglutaminases (TGases) are calcium- and thiol-dependent enzymes that modify proteins by catalyzing the formation of an isodipeptide crosslink between an .epsilon.-NH.sub.2 of a lysine and the .gamma.-amide of a glutamine residue (1-4). In mammals, five distinct TGases are known to exist: a membrane-associated activity first discovered in keratinocytes of about 92 kDa, TGase1, which is now known to be widely expressed; an ubiquitous "soluble" or "tissue" activity of about 80 kDa termed TGase2; a soluble pro-enzyme activity of about 77 kDa, known as the "epidermal" or "hair follicle" TGase3 (see, e.g. Kim, H. -C., et al., J. Biol Chem. 265:29171-21978 (1990)); an inactive TGase-like protein of about 75 kDa, band 4.2, which is an ubiquitous constituent of the subplasma membrane of most eukaryotic cells (see Sung, I.A., et al., Proc. Natl. Acad. Sci. U.S.A. 87:955-959 (1990)); and the catalytic subunit of the blood clotting factor XIII of about 77 kDa (see, e.g, Takahashi, N., et al., Proc. Natl. Acad. Sci. U.S.A. 83:8019-8023 (1986)). Curiously, all but the latter member of this family are expressed in terminally differentiating epidermis.
Several early studies reported a soluble protein of about 50 kDa from both epidermal and hair follicle tissues of the guinea pig (see, e.g., Chung, S. -I. and Folk, J. E., Proc. Natl. Acad. Sci. U.S.A. 69:303-308 (1972)), but more rigorous biochemical and cell biological analyses revealed that it is in fact a proenzyme, of molecular weight about 77 kDa, which becomes active upon proteolytic cleavage into a 50 kDa (amino terminal) and 27 kDa species (Negi, M., Colbert, M. C. and Goldsmith, L. A., J. Invest. Dermatol. 85:75-78 (1985)). While newer work has shown that these fragments are not normally separated upon activation (Kim, H. -C., et al., J. Biol. Chem. 265:29171-21978 (1990)), the fact that the isolated 50 kDa fragment can retain catalytic activity was the source of confusion in earlier studies. Furthermore, despite earlier work, it is now generally agreed that the epidermal and hair follicle pro-enzyme species are the same (Lichti, U., Ann. N.Y. Acad. Sci. 642:82-99 (1991)).